I use two ext disks. An Iomega 160GB USB 2.0 and a Maxtor 300GB Firewire. Both are good, though not bulletproof, but as backup devices, with the original data held in the original location, I am very happy with them.
Do check that you will get some decent backup software with a drive, as it makes a breeze out of the otherwise boring job and also, that whilst it is possible to do it all manually, that what you are after with backup software, is consistency.
Conan
PS If you are after a small and transportable system, Saab D mentioned a worthwhile one, but if you have a digital camera or PDA, then a 4GB Compact Flash/ SD/Whatever card can do the biz as well. Think I would avoid the microdrive like the plague though. Tiny, fragile and with lots of moving parts. Not a good recipe in my book.